Conventionally, an irradiation planning apparatus for use in particle beam therapy, typified by proton beam therapy and carbon ion therapy, assumes a human body approximately to be water having a different density, and applies a dose distribution obtained by a measurement in water to a human body, which is an inhomogeneous medium, to calculate a dose distribution in a body (see Patent Literature 1). In practice, by using a conversion table previously prepared for each CT apparatus and for each imaging condition, pixel values of a CT image (CT values) of a patient are converted into a stopping power ratio that represents an effective density of a material for a particle beam, and thus a human body is expressed as water having a different density. Here, a CT value represents an effective radiation source attenuation coefficient of a material with respect to an X-ray.
However, a human tissue differs from water. Therefore, a rate of nuclear reaction initiated, in a body, by incident particles differs from a rate of nuclear reaction initiated, in water, by incident particles. The existence of a difference between these rates of nuclear reaction means that the number of the incident particles reaching almost the end of stopping range differs between in a body and in water.
Here, the proportion of the incident particles that reach almost the stopping location has a direct effect on the height of a Bragg peak. Therefore, an error due to a difference between nuclear reaction in water and in a body occurs in the dose distribution of an irradiation plan that has been developed by applying, to a human body, a dose distribution obtained by a measurement in water. In addition, the magnitude of this error varies from patient to patient and depending on the beam direction.
However, no practical algorithms have yet been reported that correct this error.
Meanwhile, absolute dose measurement performed for quality assurance often uses a solid phantom, due to easy handling, as a substitute for water, which is a standard material. In this case, it is reported that a dose error occurs from a difference in composition (hence, a difference in the probability of nuclear reaction) between water and a solid phantom (see Non-Patent Literatures 1 and 2).
In order to correct this dose error, absolute dose measurement is performed such that a factor for converting a measured value in a solid phantom into a value that would be measured in water (fluence correction factor) is initially determined by using Monte Carlo calculation, or on the basis of actual measured data. Using this factor, measured values in a solid phantom are converted into dose values in water (Non-Patent Literatures 3 and 4).
Ideally, a dose error occurring in calculation of a dose distribution in a patient body can also be corrected using a fluence correction factor.
However, measuring the correction factor in an actual patient body is not possible. In addition, calculation of a correction factor for each patient and for each beam by using Monte Carlo calculation is impractical as well.